A lithium secondary battery is widely used for a portable electric device, a personal computer and the like, but there are requirements for further increases in energy density and prolonging battery life, and it has been a problem to balance them.
As a technique for increasing the energy density of a lithium secondary battery, silicon-based materials which are negative electrode active material having high capacity has been studied. However, there has been a problem that the silicon-based materials are inferior in life characteristics to conventional graphite-based materials.
As a cause of the deterioration of the lifetime characteristic of the silicon-based negative electrode material, it has been reported that cracks of active material particles occur due to expansion and contraction caused by insertion and desorption of Li associated with charge and discharge. In addition, there have been problems such as that gas is generated by reacting with the electrolyte solvent on the negative electrode active materials and capacity of the battery is reduced. Studies have been conducted whose objective is to reduce reactivity between silicon-based negative electrode materials and electrolyte solutions. For example, Patent Literature 1 describes an example in which a fluorinated ether is used as an electrolyte solvent in a secondary battery having a negative electrode containing silicon. Patent Literature 2 describes an example in which a fluorinated phosphoric acid ester is used as an electrolyte solvent in a secondary battery having a negative electrode containing silicon.
Further, for the purpose of improving the life characteristics of secondary batteries, combinations of compounds used for electrolyte solutions have been investigated. Patent Literature 3 describes an electrolyte solution containing a fluorine-containing open-chain ether and non-fluorine cyclic carbonate, and also describes that phosphoric acid esters may be blended for improvement of flame retardancy. Patent Literature 4 describes an electrolyte solution for a lithium ion secondary battery comprising a fluorine-containing ether, at least one fluorine-containing solvent selected from the group consisting of fluorine-containing cyclic carbonates and fluorine-containing lactones, and at least one non-fluorine carbonate selected from the group consisting of non-fluorine cyclic carbonates and non-fluorine open-chain carbonates.
Patent Literature 5 describes that, with respect to a lithium secondary battery having a positive electrode containing a positive electrode active material that operates at high potential of 4.5V or more versus lithium, a secondary battery having high energy density with improved cycle characteristics can be obtained by using an electrolyte solution containing a fluorine-containing phosphoric acid ester.